Understanding of Regional Air Pollution over China using CMAQ: Model Evaluation and Process Analysis

The U.S. EPA Models-3 Community Multiscale Air Quality (CMAQ) modeling system with the process analysis tool is applied over China, where the emissions of air pollutant are the largest in the world, to study the seasonal variations of air pollutants and their formation mechanisms. Higher 1-hr ozone (O3) level and troposphere ozone residual (TOR) occur in Jul. and Apr., and higher PM10, AOD, surface SO2, NO2 and column NO2, CO occur in Jan. and Oct. Compared with data calculated from Air Pollution Index (API) data, simulated PM10, surface sulfur dioxide (SO2) and NO2 concentrations are significantly underpredicted with NMBs up to -55%, -62% and -55%, respectively, except SO2 in Oct., due likely to uncertainties from emissions, meteorology simulation and calculation from API data. Max. 1-hr O3 concentrations in Jan and Jul were slightly overpredicted with NMBs about 10% and 20% in Jan and Jul at 36-km and good agreement was found in Apr and Oct. Simulated column variables could basically capture the high concentrations over eastern China with underpredictions in AOD in Apr and Jul, TOR in Jul, and column NO2 and CO in Apr and overprediction in AOD in Jan and Oct, column NO2 in Jan and CO in Oct over eastern China.

The IPR analysis shows that vertical and horizontal transports are most influential factors for both O3 and PM10. They are the main sources of O3 at most sites and could either increase or deplete PM10 concentrations significantly. In addition to dry deposition, gas-phase chemistry is the dominate process for O3 destruction, particularly at urban sites. Cloud processes contribution to PM10 is not significant at most sites. Aerosol processes contribute more to the formation or destruction of NO3- and second organic aerosols. O3-NOx-VOC regime analysis suggests that NOx-limited atmosphere covers almost entire China in summer except several metropolitan areas such as Beijing, Shanghai, Tianjing, and several cities in the Pearl River Delta area where the VOC-limited condition dominates through entire year. The O3 chemistry over the North China Plain and northeastern China changes from NOx-limited condition to VOC-limited chemistry in winter. Apr and Oct are the transition periods when some areas over eastern China experience conversions between VOC-limited and NOx-limited condition.